Alkali metals

Soft, silvery metals so eager to react that they fizz, flare and sometimes explode in plain water.

Drop a small piece of sodium into a bowl of water and it skids across the surface, hissing and fizzing as it gives off hydrogen gas. Do the same with potassium and the hydrogen catches fire with a lilac flame. This is the alkali metal family, the six elements in the first column of the periodic table: lithium, sodium, potassium, rubidium, caesium and francium.

Hydrogen also sits at the top of group 1, but it is not an alkali metal. It is a gas, it is a non-metal, and it usually shares electrons rather than giving one away. It is placed there because, like the alkali metals, it has a single electron in its outer shell. The name alkali comes from the fact that these metals react with water to make alkaline solutions, the chemical opposite of acids.

What they have in common

Every alkali metal has one electron in its outermost shell. That single electron is held only loosely, so the atom gives it up easily to form an ion with a charge of plus one. Losing one electron is easy, so the metals are very reactive. Because they react so readily, they are never found as pure metal in nature. Instead they turn up locked inside compounds such as rock salt, which is sodium chloride.

They are also unusually soft for metals. Sodium and potassium can be cut with an ordinary knife, revealing a shiny silver surface that dulls within seconds as it reacts with oxygen in the air. They are light too. Lithium, sodium and potassium all have densities below that of water, which is why they float during the water demonstration. In the laboratory they are stored under oil to keep air and moisture away.

Their compounds are almost always white or colourless solids that dissolve well in water. Alkali metals give off characteristic colours when heated in a flame: lithium burns crimson, sodium a strong yellow-orange, and potassium a pale lilac.

How they change down the group

Going down the column from lithium to caesium, each atom gains an extra shell of electrons. The outer electron ends up further from the nucleus and is shielded by more inner electrons, so it is even easier to remove. This is why reactivity increases down the group. Lithium fizzes gently in water, sodium reacts briskly, potassium bursts into flame, and rubidium and caesium react violently enough that the demonstration is done only on video.

Melting points fall in the same direction. Lithium melts at about 180 degrees Celsius, sodium at about 98 degrees and potassium at about 64 degrees. Caesium melts at roughly 29 degrees Celsius, so it would become a liquid on a warm day. Francium, the heaviest member, is expected to be similar. As the atoms get bigger, the metallic bonding that holds them together weakens, so less heat is needed to pull them apart.

Where you meet them

Sodium is the one you meet every day, as the sodium chloride in table salt and as the sodium ions your nerves use to send signals. Yellow street lights and the yellow flash when salt water hits a gas flame both come from sodium. Potassium is essential to plants, so it is a major ingredient in fertilisers, and it is the reason bananas are often mentioned in diet advice.

Lithium has become one of the most sought-after elements in the world because it is at the heart of rechargeable batteries in phones, laptops and electric cars. Its compounds are also used as medicines for some mental health conditions. Rubidium and caesium are rarer. Caesium's most famous job is in atomic clocks, where the steady vibration of its atoms defines the length of a second. Francium is so rare and so radioactive that it has no practical uses and has only ever been studied in minute quantities.

Odd ones out

Lithium is the family rebel. It is the hardest and has the highest melting point, and it reacts with water far more calmly than its size would suggest. It is the only alkali metal that reacts directly with nitrogen gas, and several of its compounds behave more like those of magnesium in group 2 than like those of sodium. Chemists call this a diagonal relationship.

Francium is odd in a different way. It exists on Earth only as a fleeting product of radioactive decay, and any sample decays away within minutes. Its chemistry has been predicted far more than it has been measured, and some calculations suggest it may not be quite as reactive as caesium, because its electrons are moving so fast that they are held a little more tightly than the simple trend predicts.

The numbers

Melting points fall down the list, from Lithium at 180 °C to Francium at 27 °C.

Density in g/cm³ (g/L for gases). Values for synthetic elements are predictions.

Did you know?

• Lithium, sodium and potassium are all less dense than water, so they float while they react with it.
• Caesium melts at about 29 degrees Celsius, which is close to room temperature on a hot day.
• Hydrogen sits at the top of group 1 but is a non-metal gas, not an alkali metal.
• Alkali metals are stored under oil because they react with both oxygen and water vapour in the air.
• The vibration of caesium atoms is used to define the length of a second in atomic clocks.
• Francium is one of the rarest naturally occurring elements, existing only briefly as a decay product.

Frequently asked questions

Written by Anthony. Data from Periodic-Table-JSON (CC BY-SA).